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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, November 20th, 2023 - 7:00AM
Expires
Tue, November 21st, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE above 1000′ today. There is a lingering chance of triggering a large avalanche failing near the ground on weak snow buried around 3-4′ deep, and the odds of finding a dangerous setup increase at higher elevations. There are also isolated slopes harboring a weak layer of buried surface hoar that may produce avalanches around a foot deep. The danger is LOW below 1000′.

Summit Lake: The snowpack structure in the Summit Lake area is similar to Turnagain Pass, with multiple weak layers that make it possible a person could trigger a large avalanche.

Special Announcements

Chugach State Park: The National Weather Service has issued a Special Weather Statement for the Cook Inlet area today for strong winds that are expected to impact the area. Expect to see increasing avalanche danger in CSP as strong winds make sensitive wind slabs and load a snowpack with buried weak layers. We will begin issuing weekend avalanche outlook summaries for this area starting Dec. 1.

Danger Rating Outlook – This season we’re introducing an avalanche danger ‘outlook’ for the next day. This tool aims to help assess the danger trend and hopefully help plan your outings more effectively

Mon, November 20th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Tue, November 21st, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Tue, November 21st, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Recent Avalanches

A skier triggered an avalanche in the Goldpan area two days ago, which we suspect failed on the layer of surface hoar that was buried on 11/13. More details in this observation.

Glide Avalanches: Multiple observers reported continued glide avalanche activity yesterday from Girdwood to Summit Lake (details here and here). Most of these have released within the past week.

A fresh glide avalanche in the Fresno area observed yesterday. Photo: Hannah Smith, 11.19.2023

Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

While some parts of Southcentral will be getting hit hard by northerly winds today, it is looking like our core advisory area around Girdwood and Turnagain Pass will see another day of quiet weather. The main avalanche concern for today will be the lingering possibility of triggering an avalanche on one of two buried weak layers of snow. While these are both lumped into our ‘Persistent Slab’ problem, they have some very different characteristics.

The older snow that was buried by the 11/9 storm produced a large avalanche just over a week ago in the Goldpan area behind Magnum Ridge. This layer exists above around 3000-3500′ and becomes more problematic at higher elevations. The most likely place to encounter this layer will be in steep and rocky terrain in these upper elevations. If you haven’t already checked it out, be sure to take a quick look at this observation to see what that layer is capable of.

The other layer of concern is a layer of surface hoar that was buried on 11/13 and likely was the culprit for another skier-triggered avalanche in the Goldpan area two days ago (details here). The distribution for this layer is trickier to nail down. Despite seeing widespread surface hoar just before it was buried, it only appears to remain reactive on isolated slopes. This spotty distribution is common for buried surface hoar problems, and it really requires a slope-by-slope assessment if you are trying to get into steep terrain. The best way to manage the problem entirely is by avoiding steep and consequential terrain, especially at higher elevations where the weakest snow developed before it was buried a week ago.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

We continue to see reports of glide avalanche activity from Girdwood to Summit Lake. Be sure to limit any time spent below open glide cracks, since the timing of glide avalanches is impossible to predict.

Weather
Mon, November 20th, 2023

Yesterday: Temperatures barely made it out of the single digits in most areas yesterday, with clear skies and calm to light winds around 5 mph. There were some lingering valley clouds along the Turnagain Arm, with no precipitation.

Today: Temperatures remain in the single digits above and below zero this morning, but will start to climb through the day into tonight. Expect to see daytime temperatures in the low to mid teens F, and continuing to rise into the low to mid 20’s F tonight. Cloud cover will increase through the day as well, with a trace of precipitation possible tonight and rain levels around 300-500′. Winds should remain light for Girdwood, Turnagain Pass, and Summit Lake, but will be 15-25 mph in Seward with gusts up to 50 mph in the front range.

Tomorrow: Light precipitation will begin tomorrow and continue through the week. We will likely only see 1-2″ snow tomorrow, with rain levels increasing to 1100-1300′ through the day. Winds should remain light out of the east around 10 mph, with high temperatures in the high 20’s to low 30’s F, and overnight lows in the low to mid 20’s F.

PRECIPITATION 24-hour data (6am – 6am)

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 6 0 0 32
Summit Lake (1400′) -4 0 0 22
Alyeska Mid (1700′) 7 0 0 38
Bear Valley – Portage (132′) 6 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)
Seattle Ridge (2400′)
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.